A conventional fully hydraulic steering system is illustrated in FIG. 8.
In the same figure, when a steering wheel H is operated from a straight position H.sub.2 to a left turning direction H.sub.1 or a right turning direction H.sub.3, a directional control valve b is switched from a straight position b.sub.2 to a left turning position b.sub.1 or a right turning position b.sub.3 so that a pressurized oil from a hydraulic pump A flows through a steering flow control valve B, the directional control valve b and a metering unit C and then returned to the directional control valve b. The pressurized oil thus returned to the directional control valve b enters a head side Dh of a left steering cylinder D and a bottom side Eb of a right steering cylinder E, in case of turning the steering wheel leftward and enters a bottom side Db of a left steering cylinder D and a head side Eh of a right steering cylinder E in case of turning the steering wheel rightward so that bodies F and G are turned relative to each other. The pressurized oil which entered the left steering cylinder D and the right steering cylinder E are returned to a tank T through the directional control valve b. In the same figure, designated at C is a working valve and P.sub.1, P.sub.2 as shown by two dotted chain lines illustrate that the directional control valve b and the metering unit c are operated by a steering wheel H.
In the conventional fully hydraulic steering system, since the steering wheel position is deviated due to leakage of internal oil in each hydraulic circuit, there is disclosed such a technique to compensate the position deviation as disclosed in Japanese Patent Laid-open Publication No. 60-261779. This technique is summarized as follows. The steering wheel position deviation is compensated by calculating a difference between an output of a steering wheel turning angle detecting means and an output of a steering cylinder stroke detecting means by a controller supplying a compensation signal to a solenoid operated directional control valve when an output difference therebetween exceeds a given value, draining oil from a drain oil line and racing the steering wheel so that the steering wheel position may match with the steering cylinder stoke.
However, there were the following serious problems in the conventional steering wheel position compensator.
(1) In case that the compensation operation is carried out when the position deviation occurred, the oil supplied from a steering unit (combination of the directional control valve b and the metering unit c as denoted in FIG. 8) to a steering cylinder is drained and a steering wheel is raced and compensated in the position. As a result, since the steering wheel is raced while the steering cylinder is not operated, a dead zone thereof increases so that there was a likelihood of occurring meandering when the car body travels straight.
(2) There occurred the inconvenience that the compensation was not effective when the steering wheel position is gradually deviated when the steering wheel is pressed by hands of an operator just before the directional control valve of the steering unit is operated as denoted b in FIG. 8 so as to prevent the car body from deviating slightly leftward or rightward when the car body travels straight.
It is therefore an object of the present invention to provide a steering wheel position compensator solving the problem of the conventional steering wheel position compensator of fully hydraulic steering system, improving the operation property and assuring reliability and safety with low cost.